The present invention relates to solar cells, as for example multijunction solar cells formed of III-V semiconductor alloys. The present invention is related to the invention set forth in provisional patent application Ser. No. 61/446,704 filed Feb. 25, 2011 and incorporates by reference the entirety of what is set forth in that application.
Solar cells typically utilize a window layer on top of an emitter to passivate the emitter surface and reflect back minority carriers to inhibit surface recombination that reduces efficiency. A window layer is part of the active semiconductor structure but may also constitute part or all of an antireflection coating. A conventional dielectric antireflection coating is understood not to be a window layer, although it may be optically coupled to or integrated into a window layer. (Additional transparent coverings may be provided for environmental protection.)
Multijunction solar cells formed primarily of III-V semiconductor alloys are known to produce solar cell efficiencies exceeding efficiencies of other types of photovoltaic materials. Such alloys are combinations of elements drawn from Columns III and V of the standard Periodic Table, identified hereinafter by their standard chemical symbols, names and abbreviations. (Those of skill in the art can identify their class of semiconductor properties by class without specific reference to their column.) The high efficiencies of these solar cells make them attractive for terrestrial concentrating photovoltaic systems and systems designed to operate in outer space. Multijunction solar cells with efficiencies above 42% under concentrations equivalent to several hundred suns have been achieved.
Historically, the highest efficiency solar cells have consisted of a monolithic stack of three subcells, grown epitaxially on germanium (Ge) or gallium arsenide (GaAs) substrates. The top subcell has active layers made of (Al)GaInP, the middle one of (In)GaAs, and the bottom subcell includes the Ge substrate or consists of a III-V material. The foregoing nomenclature for a III-V alloy, wherein a constituent element is shown parenthetically, such as Al in (Al)InGaP denotes a condition of variability in which that particular element can be zero.
Referring to FIG. 1, a conventional multijunction solar cell 10 comprises a stack of two or more subcells 12, 14, 16, which are typically connected by tunnel junctions 18, 20. Tunnel junctions 18, 20 typically comprise heavily doped n- and p-type semiconductor layers (not shown in detail) They are well known to those skilled in the art and need no further description here. FIG. 1 is a schematic cross-section showing an example of a generalized multijunction solar cell 10 with three subcells 12, 14, 16, interconnected by tunnel junctions 18, 20, with an anti-reflection coating 22 and an electrical contact layer 24 on top, and a supporting structure 26 that may be formed as a substrate or handle and back contact layer on the bottom. FIG. 1 is not drawn to scale.
As illustrated in FIG. 2 representing a structure of the prior art, each of the subcells 12, 14 and 16 comprises several associated layers, including a window layer 28, an emitter 30, a base 32 and a back surface field (BSF) structure 34. The window layer 28 and emitter layer 30 will be of one doping polarity (e.g., n-type) and the base layer 32 and back surface field layer 34 are of the opposite polarity in doping type (e.g., p-type), with a p-n or n-p junction formed between the base 32 and the emitter 30. If the base 32 contains an intrinsic region (not shown), then it may be considered a p-i-n or n-i-p junction, as is well known to those skilled in the art. The name of the given subcell is considered to be the name of the material comprising the base 32. For example, a subcell with the materials shown in FIG. 2 would be denoted an InGaP subcell.
When speaking about the stacking order of the subcells from top to bottom, the top subcell is defined to be the subcell that is closest to the light source during operation of the solar cell, and the bottom subcell is furthest from the light source. Relative terms like “above,” “below,” “upper,” and “lower” also refer to position in the stack with respect to the light source. The order in which the subcells were grown is not relevant to this definition.
As discussed above, each conventional subcell as discussed above typically comprises a window layer, emitter, base and back surface field (BSF), and may or may not include other layers. Those skilled in the art will also recognize that subcells may also be constructed without all of the foregoing layers. The window layer and the BSF serve to reflect minority carriers from the surfaces of the emitter and base layers, respectively, and are well known to be critical to high efficiency carrier collection. The materials and doping levels used for the window layer are chosen such that the band alignment produces a large energy barrier to the minority carriers with a minimal barrier for majority carriers. This allows majority carriers to diffuse through the window layer, while minority carriers are reflected. It is important that the interface between the window layer and the emitter be very high quality, so as to minimize the minority carrier surface recombination velocity. The window layer also has a higher band gap than the adjacent emitter, in order to minimize its absorption of incident light. In subcells other than the top subcell, the band gap of the window layer may be high enough that it absorbs a negligible fraction of the light reaching that subcell. In that case, the majority of photons with energies above its band gap would have been absorbed by the upper subcell(s).
For the top subcell, however, the window layer can be a major source of current loss. The top subcell window layer absorbs a fraction of the incident light in the solar spectrum that is above its band gap and generates electron-hole pairs, or photocarriers. These photocarriers are not collected with high efficiency due to the high surface recombination velocity for minority carriers at the top of the window layer, and the low minority carrier diffusion lengths that are common in window layer materials. The window layer of lower subcells may also be a source of loss if the upper subcell(s) do not absorb all light above the band gap(s) of the active layers.
Intrinsic material lattice constants as used hereinafter are defined as the lattice constants, or lengths of the edges of the unit cell, of a freestanding crystal of the given material. Typically, lattice constants (e.g., a, b and c) are defined in 3 reference directions (e.g., x, y and z) for a given crystal structure, wherein z is the direction of growth. The given crystal structure specifies the angles between the reference directions. For cubic material such as GaAs, all three lattice constants are the same and a single intrinsic material lattice constant is typically used. In the instances of heteroepitaxy, if a semiconductor material has substantially different intrinsic material lattice constants in the plane perpendicular to the direction of growth (a0 and b0) than the lattice constants of the underlying layers on which is grown (a1 and b1), but shares the same angle between the reference directions in that plane, the material will initially adopt the lattice constants of the underlying layers. The semiconductor material is strained, and the degree of strain is proportional to the difference between its intrinsic material lattice constants and the adopted lattice constants. The strain in the plane perpendicular to the direction of growth, which may also be considered the “in-plane” strain, can be defined as, which can also be expressed as a percentage when multiplied by 100:
      ɛ    xy    =            1      2        ⁢          (                                                  a              1                        -                          a              0                                            a            0                          +                                            b              1                        -                          b              0                                            b            0                              )      
The above discussion has assumed that the plane perpendicular to the direction of growth contains the x and y reference directions for the crystal structure. Because this is often not the case, such as for off-axis growth, we instead talk about the intrinsic material lattice spacing (a0′ and b0′) of a freestanding crystal in the plane perpendicular to the direction of growth (the x′y′-plane) compared to the lattice spacing of the underlying layers in that plane (a1′ and b1′). When the angles between a0′ and b0′, and a1′ and b1′, are the same, the strain in the plane perpendicular to the direction of growth is analogous to the above definition:
      ɛ                  x        ′            ⁢              y        ′              =            1      2        ⁢          (                                                  a              1              ′                        -                          a              0              ′                                            a            0            ′                          +                                            b              1              ′                        -                          b              0              ′                                            b            0            ′                              )      
A more complicated expression is needed when the angle between a0′ and b0′ is not the same as the angle between a1′ and b1′.
As the thickness of such a semiconductor layer with in-plane strain ∈xy is increased, the strain energy increases until a critical thickness is reached, at which point it becomes energetically favorable to relax and relieve strain through dislocations. The critical thickness depends upon many factors, including the materials involved, the substrate and/or underlying layers, growth technique and growth conditions. Below the critical thickness, the semiconductor layer is considered pseudomorphic, or fully strained. The semiconductor layer is considered fully relaxed when sufficient dislocations have formed that the layer has been essentially restored to its intrinsic material lattice constants (a0, b0, and c0). In general, layers may be fully strained, fully relaxed, or partially strained and partially relaxed when grown on top of a substrate or layers with substantially different lattice spacing in the plane perpendicular to the direction of growth.
For III-V multijunction solar cells based on GaAs or Ge substrates with emitters that are n-type and base layers that are p-type (i.e., “n-on-p” configuration), the top subcell typically has a window layer comprised of a single AlInP layer, with an (Al)InGaP emitter and base. Al0.52In0.48P is the material of choice for the top window layer, as it is the III-V material with the highest direct band gap (˜2.3 eV) that has substantially the same intrinsic material lattice constants as GaAs or Ge. However, Al0.52In0.48P absorbs a significant fraction of the incident photons with energies above its direct band gap (the fraction depending on the layer thickness), and it collects the photocarriers generated by these photons with low efficiency. More than 1 mA/cm2 under the AM1.5D or AM1.5G spectra may be lost (i.e., not collected) in this Al0.52In0.48P layer.
It is advantageous to minimize the light absorbed in the window layer due to the typical low collection efficiency for photocarriers in the window layer. One approach to minimize the absorbed light in AlInP window layers has been to increase the fraction of Al in the AlInP. This results in a window layer with a higher band gap that absorbs less light and therefore transmits more light to the emitter and base of the cell, which have higher photocarrier collection efficiencies. However, this window layer has an intrinsic material lattice constant that is sufficiently different from that of the underlying layers as to produce either high levels of in-plain strain ∈xy and/or relaxation via dislocations in the window layer.
High levels of strain or dislocations in the window layer at its interface with the emitter may increase the surface recombination velocity for minority carriers, lowering the collection efficiency in the emitter, and thus decreasing the overall efficiency of the solar cell. Thus, some of the gains in current created by increasing the fraction of Al in an AlInP window layer may be offset by an increased minority carrier surface recombination velocity. What is needed is subcell structure that decreases light absorption in the window layer while preserving a high quality window-layer/emitter interface.